94 WILLIAM BATESON. 
_ the anterior wall of the gut (Stage E to F) runs at right angles 
to the long axis of the body. As will afterwards appear this 
feature is of importance. In longitudinal section, a commenc- 
ing differentiation between the branchial and digestive region 
is perceptible. The cells of the former are columnar, while 
those of the latter have irregular ameeboid processes which 
give the inner wall of the gut an irregular contour. The anus 
is not yet formed. 
In the front end of the third segment of the larva (Stage F), 
anterior to the ring of cilia, the sides of the gut give rise to a 
pair of dorso-lateral evaginations ; these pouches are the first 
indications of the gills. No change has occurred in the skin 
covering them. Subsequently they come in contact with the 
skin, the walls fuse and then a perforation is formed through 
the fused portion, apparently occurring by a process of degene- 
ration of the tissue. Fig. 29 is from a section taken through 
the side of one of these evaginations. The subsequent appear- 
ances are shown in figs. 41 and 42, which are from an older 
larva. 
Notochord.—In the later stages of F and G in the 
anterior dorsal wall of the gut, arises a most remarkable struc- 
ture. For reasons which will appear when its later develop- 
ment and fate is considered, I propose to compare this organ 
with the notochord of the Chordata, and by this name it will 
be subsequently spoken of. 
In Stage E it was stated that the anterior wall of the hypo- 
blast came vertically to join the skin at the mouth (figs. 7 and 
10). As, however, development proceeds, the dorsal wall of 
the pharynx becomes partly constricted from the remainder 
(figs. 20 and 22). As this process of separation of the dorsal 
wall proceeds, the part so separated grows forwards so that it 
comes to project slightly in front of the anterior end of the gut 
(fig. 30). By this means a hypoblastic tube is formed dorsal 
to the gut, with a lumen which opens into the archenteric 
cavity (figs. 21, 22, and 30). 
Mesoblast.—The lining of the anterior body cavity in 
Stages E and F is composed of rounded cells arranged in con- 
